This invention relates to an elastomeric polymer of a diene monomer to which polystyrene chains are attached. It also relates to such a polymer dispersed in polystyrene to provide a composition having a substantially greater impact strength than previously described polystyrene resins and also improved aging characteristics.
Regular polystyrene resin, made solely from styrene monomer, is hard but brittle, has a tensile strength of approximately 6500 pounds per square inch, a percent elongation of approximately 2% and an Izod impact strength of approximately 0.3 foot-pounds per square inch. In order to improve the impact strength, elongation and flexibility of polystyrene resins, a number of methods have been employed in the past.
One such method is to blend the polystyrene resin with elastomeric polymers of conjugated dienes such as natural or synthetic rubbers.
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Another method has been to chemically react a small proportion of such elastomeric polymers with a large proportion of styrene monomer. These products have been referred to as "graft copolymers" and have resulted in resins of improved impact strength.
The resultant graft copolymers and the methods for their preparation vary quite widely as exemplified by the following references:
For example, in the British Pat. No. 1,044,565, a polystyrene of improved impact strength was prepared by dissolving about 2-10% of polybutadiene rubber (cis isomer content of approximately 35%, trans isomer content of about 55%) in styrene monomer together with a minute amount of anti-oxidant and mercaptan modifier. The solution was heated until there was about 40% polymerization. At this point, a peroxide type catalyst, namely tert-butyl perbenzoate, was added together with water and suspending agent to form an aqueous suspension. The heating was then continued to complete the polymerization and provide polymer beads. The impact strength as measured on an Izod machine was described as ranging from 3-6 foot-pounds per inch, depending on the percent of polybutadiene rubber. If a higher cis isomer content of polybutadiene was used, the product had a lower impact strength but better moldability.
A number of studies have been made of the chemistry of the graft copolymerization of rubber and styrene. However, there is insufficient information on the chemistry of the graft reaction, the desirable chemical species to attain, or the exact relationships of these factors to the physical properties of the graft copolymer. It is generally agreed that at about 2 to 25% styrene monomer conversion there is an "inversion point" wherein the rubber/styrene solution phase inverts to become the inner phase while the polystyrene/styrene solution component appears as the continuous phase. Precipitated rubber particles appear and are dispersed throughout the polystyrene. An optimum rubber particle size appears to be 2-15 microns but the size formation appears to depend on a number of factors such as elastomer type, concentration and temperature. Cross-linking is generally adverse to both impact strength and elongation.
The principal catalysts that have been used in graft copolymerization have been peroxy catalysts such as benzoyl peroxide. Certain azo catalysts such as azo-bisisobutyronitrile have been suggested as catalysts for polymerization of styrene. However, as pointed out in U.S. Pat. No. 3,644,587 azo-bisisobutyronitrile produces no grafting of polystyrene on rubber and a peroxide catalyst is required.
Although polystyrene resins of improved impact strength have been provided by the methods described in the prior art, the resultant products are usually accompanied by one or more deficiencies, such as inadequate aging characteristics, difficulty in molding, insufficient tensile strength and inadequate flexibility and solubility.
One object of the present invention is to provide a novel elastomeric polymer of a diene monomer to which polystyrene chains are attached and which is characterized in that its dispersion in a matrix of polystyrene provides compositions of improved impact strength, tensile properties and solubility.
Another object of the present invention is to provide polystyrene resins of improved impact strength accompanied by good aging resistance, flexibility, moldability, tensile properties and substantially complete solubility.
A further object of this invention is to provide a commercially feasible method for producing such resins.